First, you have to forget some of what you have heard about
optics and dome port design. There are lots of websites and books that explain
the virtual image. The virtual image
is at a distance away from the dome port that is approximately 2 times the diameter of
the outer dome port. What most people don't explain is that the virtual image
is NOT flat, but covers the whole angle of view of the lens. If you don't
believe me then try on a Hydro-Optix mask. The image
in the mask comes from all around
the view of your eye, not in a flat plane.

FACT:

1. The water/air interface of a dome
becomes the first element of the lens.

2. For an image to be the sharpest it can be, the
center of the dome radius must be at the nodal point of your particular lens.

Therefore, the front and sides of your
dome must be
equidistant away from the nodal point of the lens.

3. The nodal point of a lens is the point within the
lens where all the angles of light converge. People that shoot panoramas
know about the nodal point, so it is easy to search the internet to find more
information on nodal points. For more information on nodal points go
HERE.

In the image above, the red, green and blue
lines mark the convergence to the lens' nodal point. When the distances
are not equal, the lens may have trouble keeping all angles in focus.

Many people purchase very large diameter domes that are not
suited well for extreme wide angle lenses. These wide ports usually have
the center of the dome closer than the sides, which means the corners of the
image can be blurred or out of focus.

The above image shows what happens when a dome that is too wide for the lens.
The center of the dome radius is too far to the rear of the nodal point of the
lens, possibly making the corners of the image blurred.

Having a dome that is too small can have the same affect as having too large a
diameter dome. The difference being the too small port doesn't place the
focal plane as far back as a dome that is too large.

The Bare dome port assembly is optimized for the Tokina 10-17 mm
and the Nikon 16 mm F2.8 AF lenses. The center of the dome radius is
positioned exactly at the nodal point of the lens assuring sharp corners.

Look at the images on our
website to see how sharp your images
can be from corner to corner.